Pure terephthalic acid (PTA) is produced by the oxidation of paraxylene to form a crude terephthalic acid. The crude terephthalic acid is separated from the oxidation process solvent and is then purified, for example by using hydrogenation and/or crystallization techniques, to form a PTA slurry in aqueous mother liquor. The PTA is typically then separated from aqueous mother liquor in a two stage process. The first stage of separation is typically carried out in decanter centrifuges or rotary filters at a pressure of greater than 3 bara and a temperature of greater than 140° C. The damp cake leaving the first stage of separation is re-slurried in hot water before the pressure is reduced. The PTA is then separated from this water at atmospheric pressure in a second stage of separation. This second stage of separation is typically achieved using decanter centrifuges or rotary filters. The water removed in the second stage can be used to slurry CTA at the front end of the purification process. The final PTA product typically contains less than 150 ppm (w/w) of the intermediate para-toluic acid.
The two stage separation process described above can be improved by using rotary filters that allow a cake washing step to be applied during the filter cycle and hence improve the removal of soluble impurities. Reduced cake moisture versus centrifuges can also be achieved using such filters. If sufficient wash efficiency is established in the rotary filter, then an acceptable product quality can be achieved using a single stage of separation. To achieve this, the separation must be performed at a pressure of greater than 3 bara and a pressure letdown device used to allow the PTA to be transferred from the pressurized filter system to the low pressure systems downstream. This method can reduce the capital cost of the process and the manufacturing cost of the PTA.
The majority of the rotary pressure filters commercially available for this duty use a gas to displace the aqueous mother liquor and/or washing liquor. Typically the casing of the rotary filter is pressurized with an inert gas which forms a pressure driving force for displacing liquid from the filter cake. Some of this gas passes through the cake and a mixture of gas and liquid is collected in the filter drum or filter cells, depending on the rotary filter design. Some rotary filter designs allow the gas and liquids collected from different parts of the filter to be collected separately, other designs collect all gas and liquids together.
A problem with using an inert gas to create the pressure driving force is the fact that as it passes through the cake it causes evaporative cooling and precipitation of impurities from the mother liquor including the important impurity para-toluic acid. This precipitation will cause the content of impurities, including para-toluic acid, in the PTA to increase. The consequence is either the product being out of specification or an increase in operating cost due to an operational change required to maintain product quality.
U.S. Pat. No. 5,583,254 generally discloses using an inert gas containing steam as an alternative means of creating the pressure driving force, but does not disclose any concentrations of steam to use. Additionally, steam has a commercial value and therefore would be an additional operating cost without recycling the gas containing steam. The problem with recycling the gas containing the steam is that some of the impurities, including para-toluic acid, are volatile in steam and hence the steam leaving the filter would be contaminated in these impurities and would contaminate the cake if directly recycled. U.S. Pat. No. 5,583,254 generally discloses that the gas can be treated and reintroduced, but does not disclose any methods for achieving that result. The consequence of these combined problems is either the product being out of specification or an increase in operating cost due to an operational change required to maintain the product quality.
Therefore, there exists a need to further i) minimize the impurities, including para-toulic acid, in the production of a pure carboxylic acid, for example pure terephthalic acid, and ii) reduce operating costs in maintaining product quality.